1. Field of the Invention
The present invention relates to a DC power transmission system, and in particular to a DC power transmission system using a voltage source converter with a pulse-interleaving auxiliary circuit comprising a normal transformer and a 3-level half-bridge.
2. Description of Prior Art
Generally, an AC voltage and an AC current being outputted from a generator has a low voltage and a high current. The AC voltage and the AC current are subjected to a long distance power transmission in a form of a low current while maintaining a high voltage using a high voltage transformer or a ultra-high voltage transformer due to a conductor loss by the high current during the power transmission. However, the AC power transmission has a limitation in the long distance power transmission due to an inductance in the power transmission line and a capacitance between a power transmission line and a ground. In order to overcome the above-described limitation, a current source converter for converting AC to DC employing a thyristor having a large capacity was developed, thereby allowing a DC transmission.
Of the above-described DC power transmission, a high-voltage DC (HVDC) transmission system that provides power by converting an AC power generated in a power plant to a DC power to be transmitted and converting back the DC power to the AC power at a receiving point has been widely used. The HVDC transmission system allows an economical power transmission through a step-up of a voltage, which is an advantage of a conventional AC power transmission technology and is also overcoming disadvantages of the conventional AC power transmission technology.
The HVDC system employing thyristor, which has no turn-off capability at the gate, consumes reactive power from the interconnected AC system when it operates. A HVDC system using a voltage source converter, which employs semiconductor switches with gate turn-off capability such as GTO (Gate Turn-Off thyristor) or IGBT (Insulated Gate Bipolar Transistor), does not need reactive-power compensation. On the other hand, it has a capability to compensate the reactive power required in the interconnected AC system.
A voltage source converter used in the HVDC transmission system includes a PWM converter wherein each of switching elements that constitute a single bridge is operated in a PWM mode and a multi-pulse converter that generates an output waveform by combining two or more bridges using transformers.
While the PWM converter has a simple system configuration using the single bridge, a switching loss is large due to multiple switching of each of the switching elements per one AC cycle. Therefore, the PWM converter is not suitable for a large capacity system.
Moreover, while the multi-pulse converter has a small switching loss due to a single switching per AC cycle, the number of pulses should be increased in order to reduce a harmonic level of the output waveform. Therefore, various schemes are used to increase the number of the pulses of the output waveform. The simplest scheme thereof is to increase the number of the bridges of the converter and the number of the transformers coupled to an AC output terminal to increase the number of the pulses. However, this scheme is disadvantageous in that a size of the system is large and a manufacturing cost is high due to the increase in the number of the bridges and the transformers. In order to overcome the disadvantage, an auxiliary transformer is employed between the main transformer and the bridge, maintaining the number of the bridges while increasing the number of the pulses. However, a connection structure of the auxiliary transformer is complex so that a manufacturing process thereof is complicated and a reduction of the manufacturing cost is low.
Therefore, a method wherein an auxiliary circuit is inserted at a DC stage to superpose a voltage in a form of the pulse on a voltage applied to a DC capacitor to generate the output waveform. FIG. 1 is a diagram illustrating a conventional multi-pulse DC power transmission system disclosed in Korean Patent No. 10-034614.
The system shown in FIG. 1 comprises a multi-winding transformer 1 having a primary winding connected between a connection point of a ground terminal of a first converter 13 and an output terminal of a second converter 11 and a connection point of an output terminal of the first converter 13 and a ground terminal of the second converter 11 so that a difference of output voltages of the first converter 13 and the second converter 11, first and second reactors 9 for rectifying first and second currents connected to one terminal of the multi-winding transformer 1 and output terminals of the second converter 11 and the first converter 13, first and second DC dividing condensers 8, and a plurality of thyristors 2 and 3 respectively connected to a second terminal of the multi-winding transformer 1 wherein one of the plurality of the thyristors become conductive by a rising edge pulse of a primary voltage thereof. The system is manufactured to operate identical to a 24-pulse thyristor HVDC system.
FIG. 2 is a diagram illustrating another conventional 36-step converter using a DC auxiliary circuit.
The conventional 36-step converter shown in FIG. 2 is a 36-step converter including an auxiliary circuit consisting of an H-bridge and a tapped transformer, and a 12-step converter wherein a voltage generated by combining a voltage of a DC capacitor and a voltage formed by the H-bridge and the tapped transformer is provided to each 6-step bridge. An output voltage generated at each of the 6-step bridges is combined by a three-phase transformer so as to output an output waveform of 36-step.
However, the conventional arts shown in FIGS. 1 and 2 requires a special design and a manufacturing process compared to a normal transformer since the tapped transformer has the large size and a voltage ratio cannot be accurately matched. Moreover, when an inaccuracy of a winding ratio of the tapped transformer results in a lack of a symmetry of the output voltage waveform, thereby generating a harmonic.
On the other hand, the HVDC system may be classified into a point-to-point system which is a DC link type consisting of a cable or an over-head line or a combination thereof, and a back-to-back system wherein the rectifier and an inverter are placed in a converter station. Since the back-to-back system is used to connect two AC systems having different frequencies or connecting a large scale wind power generation plant to a power system, the back-to-back system should be capable of independently controlling active/reactive powers of the two connected AC systems and of controlling a bi-directional power flow. While a magnitude and a phase of an AC output voltage may be independently controlled and the active/reactive powers may also be independently controlled when the voltage source converter operates in the PWM mode, the switching loss is generated when the PWM mode is applied in case of a large capacity voltage source system.